Various types of combustion may be used in an internal combustion engine. For example, spark ignition (SI) of a homogenous mixture during the expansion stroke is one example method. This method relies on a timed spark from a sparking plug in order to achieve ignition within the combustion chamber of an air/fuel mixture. Another type of combustion is homogeneous charge compression ignition (HCCI), which occurs when the temperature of the air/fuel mixture within combustion chamber attains the autoignition temperature of the specific fuel. HCCI operations can be used to provide greater fuel efficiency and reduced NOx production under some conditions.
In some gasoline engines, the engine can perform SI operation under some conditions, and HCCI operation under other conditions in order to achieve improved overall operation. However, HCCI combustion can require the mixture to be diluted by air or combustion products (residuals), thus creating a high inlet pressure in HCCI mode (i.e. un-throttled or boosted). In contrast, SI combustion can operate with the mixture at, or close to, stoichiometric proportions. Thus at part load, inlet pressure with SI operation may be lower than atmospheric pressure to reduce the fresh air mass. As a result, during combustion mode transitions (such as at part load), a substantially rapid change of intake pressure and/or temperature is often desired.
One approach to performing a transition between combustion modes is found in U.S. Pat. No. 6,336,436. In this approach, fuel injection timing is adjusted during a transition operation wherein stratified charge combustion is performed. The stratified charge combustion is performed by injecting fuel during a compression stroke. In this manner, the intake pressure and/or temperature may be adjusted gradually over a period of multiple cycles.
The inventors herein have recognized a disadvantage with such an approach. As one example, under some conditions, utilizing stratified charge combustion may still require a substantially rapid change in intake manifold pressure. In another example, the disparate type of combustion between stratified and compression ignition operation may cause degraded performance during the transition.
In one approach, the above issues may be addressed by a method of operating an internal combustion engine having a combustion chamber with a piston and a spark plug, comprising: when transitioning between spark ignition combustion and autoignition combustion, creating a first mixture of air and fuel, adjusting an operating condition of the engine so that said first mixture of air and fuel in the combustion chamber approaches, but does not achieve, the autoignition temperature, and performing a spark from the spark plug so that at least a portion of said first mixture combusts to raise a remaining portion of said first mixture to said autoignition temperature.
In this way, it is possible to provide transitional operation in place of, or in addition to, a stratified transition. This can provide a mode of operation that is closer to compression ignition operation so that the change in conditions may be reduced during the transition. Alternatively, or in addition, additional operation may be provided during the transition so that a slower manifold pressure change can be provided. Thus, an improved transition can be provided, at least under some conditions.